Abstract

Fluornatropyrochlore is a new pyrochlore supergroup mineral species found in the intrusive alkali granite of the Boziguoer rare earth element deposit in Baicheng County, Akesu, Xinjiang Autonomous Region, China. This new mineral is found to be closely associated with microcline, albite, aegirine, a sodic amphibole, biotite, zircon, rutile, thorite, fluorite, fluocerite-(Ce), columbite-(Fe), xenotime-(Y), astrophyllite, chevkinite-(Ce), and fergusonite-(Y). Fluornatropyrochlore occurs mostly as anhedral crystals, rarely as subhedral grains, with a grain size of 0.02 to 0.25 mm. The color is brownish yellow to reddish orange. Other physical properties are: brittle, translucent to transparent, non-fluorescent, adamantine luster, light yellow streak, with Mohs hardness 4–4½, and calculated density 5.275 g cm−3. Fluornatropyrochlore is isotropic, n = 2.10(5) (589.9 nm). It has a cubic crystal structure, with unit-cell parameters determined using single-crystal X-ray diffraction as: a 10.5053(10) Å, V 1159.4(2) Å3, Z = 8, space group Fdm. The strongest ten X-ray diffraction (XRD) peaks in the powder XRD pattern are [d in Å(I)(hkl)]: 6.0741(3)(111), 3.0416(100)(222), 2.6280(38)(004), 1.8568(34)(044), 1.5820(15)(226), 1.5148(4)(444), 1.3137(2)(008), 1.2045(3)(266), 1.1726(2)(048), and 1.0712(1)(448). Chemical analyses by a combination of electron micro-probe, Fourier transform infrared spectroscopy (FTIR), and single crystal-structure refinement give its chemical composition as: Na2O 6.80, K2O 0.01, CaO 2.01, MgO 0.01, FeO 0.05, SrO 0.03, PbO 16.17, Ce2O3 4.29, La2O3 1.65, Nd2O3 0.41, Y2O3 0.42, SiO2 0.03, TiO2 1.36, UO2 5.81, Ta2O5 3.00, Nb2O5 53.42, F 3.19, Cl 0.02, ThO2, Sb2O5 0.01, ZrO2 0.01, MnO 0.04, SnO2 0.34, H2O not detected, with a sum of 98.21 wt.%. The empirical formula derived from its averaged composition is (Na1.03Pb0.34Ca0.17U0.10Th0.01Ce0.12La0.05Y0.02Nd0.01)Σ1.85(Nb1.88Ti0.08Ta0.06Sn0.01)Σ2.03O6.21F0.79 based on 7 anions per formula unit. The crystal structure was refined to an R index of 0.053 using MoKa X-ray intensity data. Site-scattering refinement analysis shows that the new mineral is characterized by F dominance at the Y site, Na dominance at the A site, and Nb dominance at the B site, based on the overall electroneutrality requirement of the structural formula.

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